A blog about the history, current state, and the future of the electric guitar and tube-based amplifier combo and how this de rigueur musical instrument that matured during the Rock N' Roll-era continues to both evolve and refuses to die.

Monday, October 26, 2015

Can The Type 85 Vacuum Tube Be Used As An Audiophile Grade Fuzz Unit?

Even though it dates back from the 1930s Golden Age of
Radio, but can the Type 85 vacuum tube work as a heart of an audiophile grade
fuzz unit?

By: Ringo Bones

To some, an “audiophile grade fuzz unit” may sound oxymoronic
because after all they are primarily designed to produce harmonic distortion at
relatively low volumes in electric guitar amplifiers. But given that vacuum
tubes and electric guitar electronics had been going hand-in-hand for over 60 years
with exceedingly good sounding results, should electric guitar electronic
effects designers explore underutilized vacuum tubes for possible use in the
hopes of designing one that offers both versatility and good sound quality?

Fuzz units or fuzz pedals as they are originally known were
originally introduced back in the 1960s and were readily embraced by rock and
Blues electric guitar players. It makes use of germanium signal diodes to clip
the electric guitar signal originally produced by the magnetic pickups to
produce a chainsaw-type distortion. Germanium semiconductor diodes have a lower
voltage drop, Vf = 0.2 volts, in comparison to silicon semiconductor diodes at
Vf = 0.6 volts. This means that the signal is clipped at a much lower level.
The sound produced by this electric guitar effect / processor is quite
versatile. It can sound warm and fuzzy to nasal and grainy. The most famous
example of this effects unit is the Dallas Arbiter Fuzz Face. Originally, they
are composed of solid-state semiconductor parts – i.e. germanium signal diodes
and germanium transistors during the 1960s to operational amplifiers by the
start of the 1980s, but will a better-sounding one be made around the Type 85
vacuum tube?

The Type 85 vacuum tube is a duo-diode triode vacuum tube in
a single enclosure designed for use as the first-stage detector, automatic
voltage control and first stage audio amplifier in 1930s AM radios / AC line
operated receivers. It was also used as the phase inverter in several 1930s era
public address amplifier designs. It is electrically identical to the octal
base 6V7. The Type 85S is a spray shield type made by Majestic. The Type 85
vacuum tube has a maximum plate voltage rating of 250 volts DC but it is
typically run at 135 volts, a maximum plate current of 8-milliamperes but
typically it is run at 3.7-milliamperes, a maximum grid voltage of -20 volts
but it is typically run at -10.5 volts, a filament or heater voltage of 6.3
volts and a filament current of 300-milliamperes. The Type 85 vacuum tube has
an amplification factor or mu of 8.3 and a transconductance or gm at 750 and it
has a plate resistance of 11,000-ohms.

The Type 85 vacuum tube is a multiunit vacuum tube that is
it is a vacuum tube containing several independently acting tubes in one
envelope. The electron stream is divided into several parts, each part being
acted upon by one set of electrodes. The top metal cap is electrically
connected to the ground and therefore does not represent a high voltage
electric shock hazard. But can the two vacuum tube signal diodes enclosed in
the Type 85 vacuum tube be used as “clipping diodes” for it to be used as an “audiophile
grade fuzz unit”?

Even though hobbyists and DIY-ers well versed on how the
Type 85 vacuum tube works are still relatively rare, Kara Chaffee of
deHavilland managed to design and produced one in current commercial production
called the deHavilland Mercury preamplifier. Although she didn’t use the two
built in vacuum tube diodes and left them unconnected. But given the Type 85
vacuum tube’s robust tone and crystalline clarity, would it be a great vacuum
tube not just for electric guitar preamplification and distortion work, but
also for acoustic guitar amplification given its robust tone can make a typical
acoustic guitar’s built in piezoelectric Fishman Transducers sound as if they
were 3,000 US dollar Condenser Tube Microphones?

Even if the Type 85 vacuum tube’s big size might intimidate the
contemporary 21st Century guitarists, there are now pack-of-gum-sized lithium polymer rechargeable
batteries with nanostructured anodes sold in Airsoft shops that can hold enough juice to power a
Vietnam War era AN / PVS-2 starlight scope with its 15,000 volt photomultiplier
tube for 15 hours straight before recharging, so battery operated fuzz pedal units
using the Type 85 vacuum tube is technically feasible this day and age. And given
that the Type 85 vacuum tube was used as a automatic voltage control unit in AM
radios during the 1930s Golden Age of Radio, with its built in vacuum tube
diodes, it could also be useful as a gold channel or audiophile grade
compressor-sustainer unit, noise gate and limiter.

2 comments:

Thanks to Kara Chaffee at deHavilland Electric for rescuing the Type 85 vacuum tube from obscurity. Given the diodes built in into the envelope of the Type 85 vacuum tube, it sort of remind me of the "transconductance amplifier circuit" of the Dolby B-type noise reduction circuit and it probably only needs the proper resistor and capacitor values to work as such. Or maybe we'll just use it to make the Fishman Transducers in those "affordable" Ovation acoustic guitars sound better.